I apologise in advance if any of my queries appear slightly basic for someone who's trying to work through Oedipus at Colonus; the Greek I have under my belt is the result of an intensive six-week summer school course, so there's a lot that have either been left out or glossed over or which has simply slipped from my mind. I'm currently working my way through Sophocles' Oedipus at Colonus for the purposes of directing it, and I thought I'd give myself a better sense of the language and meaning in the text by looking at the Greek. I'm also trying to adapt Jebb's more archaic language to a more contemporary form so that my actors will have something less opaque to work from. Unfortunately, most of the actual Greek is too advanced or elaborate for me to fully understand, but on occasion when I read something that I know I can work out, I will look at it in greater depth rather than working from a myriad of translations. I also do that for constructions or lines that seem significant or interesting.

What I'm currently curious about is, in Jebb's 1889 commentary, marked as line 74:

ὅσ᾽ ἄν λέγωμεν πάνθ᾽ ὁρῶντα λέξομεν.Jebb: In all that I speak there shall be sight.

1) ὅσ᾽, I am guessing, is a truncated form of ὅσα? I can't recall if ὅσος is declinable or not, and Goodwin isn't of any help here.

2) I'm fairly certain there is a sort of parallel construction going on with λέγωμεν and λέξομεν, with there being a tension between the subjunctive of the former and the future of the latter, but what I'm particularly confused about is why Oedipus uses the 1st person plural here rather than the 1st person singular. Is this some sort of 'royal we' that the Greeks have, or is there something else going on?

I realize this translation is rather archaic and is probably not fitting for contemporary audience, but it does stay true to the construction here:

Whate'er I speak, clear-visioned I shall speak.

As you said, ὅσ᾽ is a truncated form of ὅσα (ὅσος is declinable). A relative clause with ἄν and subjunctive represents an indefinite statement. I'm not sure whether πάντα is the antecedent or going with ὁρῶντα. Probably the former.